Scattering of photons by periodic photonic structures gives rise to a variety of interesting physical effects including manipulation of spontaneous emission, formation of forbidden photonic gaps, resonantly enhanced transmission through sub-wavelength apertures, and emission profile shaping, which can be exploited for sensor applications enabling direct label-free detection of various molecular targets in real time. It has been demonstrated that these effects can also be observed in the structures with deterministic-aperiodic (e.g., quasi-periodic) morphologies that do not possess translational periodicity. Furthermore, phenomena inherent to random structures, such as light localization, have been demonstrated in aperiodic photonic structures with high degrees of structural complexity.
The periodic-grating-based biosensors demonstrated to date usually track the change in the angular intensity distribution of the diffracted light or the frequency shift of the scattering or back-reflection resonance. Double-periodic gratings having a different period in each lateral direction, which feature resonant reflection peaks at two specific wavelengths, depending on the polarization of incident electromagnetic wave, have also been proposed for biomolecule detection using two distinct sensing modalities. A resonant reflection peak excited by one polarization of incident light has been tuned to coincide with the excitation wavelength of a fluorophore, while the shift of the peak generated by the light of orthogonal polarization has been used for label-free detection of adsorbed biomolecules. Analogously, sensors based on periodic photonic crystals with or without defects can detect the change in the refractive index of the ambient gas/liquid or the presence of infiltrated nanoparticles by monitoring the frequency shifts of high-Q factor optical resonances (corresponding to either band edge or defect modes) of the PhCs structures. Modification of the local density of states (LDOS) at selected wavelengths corresponding to the high-Q modes in periodic photonic crystals can also be used for enhancing signal from fluorophores. Finally, periodic gratings of metal nanoparticles with a period optimized to produce a sharp scattering resonance accompanied by a resonant enhancement of the near-field intensity at a selected wavelength owing to formation of photonic-plasmonic modes have been used for enhancing fluorescent and Raman signal as well as the IR absorption of biological molecules.